1. Field of the Invention
The present invention relates to a semiconductor device which comprises a semiconductor body doped with an impurity to form a carrier recombination center.
2. Description of the Prior Art
In the art of a high speed semiconductor device, it has been the practice to diffuse into a semiconductor body an impurity forming a carrier recombination center as a lifetime killer in order to attain high switching speed. For the purpose of describing the prior art relating to the lifetime killer, the mechanism of the carrier recombination will now be considered.
Carrier recombination through the recombination center is accomplished when either electrons or holes, e.g. excess electrons in a conduction band are first captured by unoccupied impurity ions which have not captured electrons and then excess holes in a valence band are captured by the impurities. In this case, if the impurity which has captured the electron releases that captured electron to the conduction band before it captures the hole, no recombination occurs. Since the rate of the release of the electron from the impurity ion increases as an energy level which the impurity defines in a band gap of a semiconductor body approaches the conduction band, the rate of carrier recombination is small when the energy level is close to the conduction band. On the other hand, if the energy level approaches the valence band, many of the impurities are filled with electrons resulting in a decrease in the amount of non-occupied impurities which can capture excess electrons. Therefore, in this case, the rate of recombination also decreases.
While the above explanation has been made in connection with electrons, the same is applicable to holes. Thus, when the energy level of the impurity lies near the center of the band gap, the rate of carrier recombination is maximum and the lifetime of the carrier is minimum. For this reason, as a lifetime killer, an impurity with a deep energy level, in other words, an impurity an energy level of which lies near a center of a band gap has been used heretofore. A typical example is gold in a silicon semiconductor. When the impurity of a deep energy level is used as the lifetime killer, the lifetime of the carrier can be shortened but there exists a drawback of increased leakage current at a junction. This can be understood when considering that the high rate of recombination of excess carriers (excess electrons and holes) also means a higher generation rate of carriers in a depletion layer near the junction. On the other hand, if one uses an impurity having a shallow energy level to reduce the leakage current at the junction, the effect of shortening the lifetime is sacrified for the reason described above. Furthermore, the rate of the emission of electrons or holes from the impurity ions increases with temperature, and since the effect of the emission of electrons or holes to the recombination is significant when the energy level is shallow and the rate of recombination decreases with the increase of the rate of emission, a device having a shallow energy level lifetime killer diffused therein has another drawback of an extended lifetime at an elevated temperature resulting in an extended reverse recovery time of the junction.
It is desirable to use, as a lifetime killer, an impurity which effectively functions only as a recombination center for the excess carrier and has no or little additional functions. However, the above requirements of short lifetime of the carrier, no extension of the life time at a high temperature, and small leakage current at the junction are competing factors in the lifetime killers which heretofore have been available, and it has been very difficult to meet the requirements simultaneously.